When carbonaceous materials are heated during a gasification process, gaseous species of varying molecular weights are released as product gas. Production of product gas via gasification of renewable resources has been a focus for researchers for decades. For this purpose, carbonaceous materials include but are not limited to biomass, waste, coal, etc.
Product gas as used herein is a mixture of hydrogen (H2), carbon monoxide (CO) and other combustible and non-combustible gases whereas the hydrogen and carbon monoxide concentrations are maximized, and can be considered as either a fuel gas where it is typically burned directly as fuel to produce heat and/or electric power or as an intermediate for multiple uses, such as synthesis of liquid fuels, chemicals, or other materials.
Carbonaceous species in product gas with molecular weights greater than benzene (MW=78) are generally classified as tars. As initially produced, these tars are reactive or problematic due to their chemical functional groups including but not limited to: hydroxyls, aldehydes, ketones, carboxylic acids, alkenes, alkynes, heterocyclic structures, in any combination, which can allow them to polymerize and thereby cause plugging, form coke or other solid deposits, cause equipment to seize, or have other deleterious effects. The presence of these reactive or problematic tars in product gas has plagued most gasification projects and has been the Achilles heel of gasification.
Capital needs for conversion of carbonaceous materials to product gas are substantial and available processes and equipment still leave much to be desired by way of efficiency of production and ease of operation and maintenance. Although the process of gasification has been practiced for decades, and many, many gasifier designs have been invented, no gasifier exists that can produce a product gas free of tar at commercial scales appropriate for economically compelling conversion of carbonaceous materials into liquid fuels, electric power, or chemicals.
What was needed was a method and apparatus that performs tar conversion to a large extent, would increase product gas production, and would increase carbon conversion, and could do so under conditions that would prevent melting, slagging, clinkering, or agglomeration of char-ash particles, and can be built at an economically viable scale and cost. As used herein, “tar conversion” or “conversion of tar” means removal, modification, or transformation of chemical functional groups within the tar species, including but not limited to: deoxygenation, hydrogenation, reforming, cracking, depolymerization, or other chemical reactions that result in less problematic tar species and/or lower molecular weight species including gases such as H2 and/or CO.
As is known in the art, Ziad Abu El-Rub, Biomass Char as an In-situ Catalyst for Tar Removal in Gasification Systems, PhD thesis dissertation, Twente University, Enschede, The Netherlands, March 2008, char-ash (also known as biochar, char, fly ash, or simply ash) can be used as a catalyst to convert tars produced in the gasification of carbonaceous material. It would be desirable to partially oxidize the residual carbon in the char-ash while avoiding oxidation of hydrogen or other valuable constituents in the product gas to generate additional CO in the product gas, generate enough heat to support/enable endothermic tar conversion reactions, enhance or improve the catalytic activity of the char-ash for tar conversion reactions, and maximize carbon conversion while simultaneously enabling smaller reactor volumes or reduced reactor temperatures, all leading to lower cost and more robust operations.